The three principal activation pathways of the complement system (the classical, lectin and alternative pathways) converge on complement peptide C3, making it a central target for drug development in the search for immune system modulators for the treatment of complement-mediated disorders. Excessive or uncontrolled activation of the complement system plays a key role in a wide range of autoimmune and inflammatory diseases. Pharmacological inhibition of C3 activation can modify all outcomes of complement cascade activation (opsonization, inflammation and membrane attack complex formation), irrespective of which complement pathway is activated.

Examples of C3 inhibitors in development, all of which are compstatin derivatives: